20 best iPhone apps to get you started

4.
Khan Academy

Khan Academy is unbelievable, Bill Gates said in 2010. The online trove of more than 3,800 education how-tos was ported to the smart phone world early last year.

Users can browse 10- to 15-minute videos showing creator Salman Khan and others doodling away on a digital black board to explain subjects from making hexaflexagons to the stereochemistry of dienophile.

Khan Academy also features new videos that help explain issues the world faces today – like a four-part series on the Greek debt crisis or the introduction to the US social security system.

Khan Academy’s videos hosted on YouTube have received more than 200 million hits.